Many accessories driven by automotive engines are equipped with a drive belt which must be maintained at the correct tension, and a variety of devices have been proposed to automatically adjust the drive belt tension. U.S. Pat. Nos. 4,249,425, 4,298,342 and 4,300,890, for example, disclose systems which have a belt tensioning mechanism actuated by oscillations of a vacuum actuator. In those systems, the vacuum actuator oscillated in response to variations in vacuum produced by a gasoline engine induction system. Thus in the absence of special provisions, those systems have not been applicable to diesel engines because diesel engine induction systems do not produce a variable vacuum signal.